fix(npm)

[jhheider]

ref npm/cli#6142

fixes#213
fixes#214
fixes#215

[what-the-diff]

• The test command was changed to include a pipe and two commands.
• A new line of code was added that checks if npx is installed on the system, which it should be since we just ran npm install in the build step above.
• Another new line of code lists all files in {{prefix}}/bin directory (which will contain our newly built tldr binary). This helps us verify that everything worked as expected during installation process by checking for presence of this file after running npm run test.